Diethyl ethoxymethylenemalonate (I) is condensed with alpha-fluoroacetamidine (II) to afford the pyrimidinone (III). The alkylation of (III) with 2-bromo-4'-isobutyloxyacetophenone (IV) gives rise to the required N-alkylated pyrimidinone (V), along with major amounts of the unwanted O-alkylated regioisomer (VI), which can be separated by chromatography. Cyclization of (V) in the presence of NaOEt provides the key pyrrolopyrimidinone compound (VII). After alkylation of (VII) with 2-fluorobenzyl bromide (VIII) in the presence of tetrabutylammonium fluoride, the resultant N-benzyl derivative (IX) is subjected to Mannich reaction with N-methyl-2-(2-pyridyl)ethylamine (X) and formaldehyde to produce the title compound.